1.8 Emergence of healing in the Antarctic ozone layer

Antarctic ozone depletion has been a focus of attention for scientists, policy-makers, and the public for over three decades. The fundamentals of polar ozone depletion are well known and driven by chlorine and bromine chemistry linked to industrial halocarbon emissions. Ozone-depleting halocarbons have been phased out under the Montreal Protocol and subsequent amendments starting in the 1990s. In response to this historic agreement, a chemically driven increase in polar ozone is expected. Ozone recovery involves multiple stages, starting with (i) a reduced rate of decline followed by (ii) a leveling off of the depletion and (iii) an identifiable ozone increase that can be linked halocarbon reduction. This third stage of recovery can be referred to as “healing”.  In Figure 1, this healing is shown for the post-2000 period for both observations (i.e., Solar Backscatter Ultra-Violet satellite (SBUV); ground-based South Pole station) and model calculations. The model calculations were carried out with the Community Earth System Model 1 (CESM1) Whole Atmosphere Community Climate Model (WACCM), which is a fully coupled state-of-the-art interactive chemistry climate model. We used the specified dynamics option, SD-WACCM, in which meteorological fields, including temperature and winds, are derived from observations. The analysis fields allow the time-varying temperature-dependent chemistry that is important for polar ozone depletion to be simulated in detail. This work included four WACCM-SD simulations for the post-2000 period: 1) all chemical, dynamical processes with volcanic eruptions (Chem-Dyn-Vol); 2) same as 1), but without volcanic eruptions (Vol-Clean); 3) repeating year 1999 meteorology showing only the impact of halogen recovery chemistry (Chem-Only); and 4) one simulation for pre-2000 period based on the Chemistry Climate Model Initiative (CCMI). This simulation includes volcanic eruptions.

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Figure 1. Monthly averaged Antarctic total ozone column for October and September, from SBUV and South Pole station observations and a series of model calculations. Total ozone data measured at the geographic South Pole are from Dobson observations (filled circles) for October (left) and balloon sondes (open circles) for September (right), when there is not sufficient sunlight for the Dobson. SBUV data for each month are compared with model runs averaged over the polar cap latitude band that is accessible by the instrument; South Pole station data are compared with simulations for 85°S to 90°S.

This work shows for the first time that chemical recovery is occurring. Observations and model calculations together indicate that healing of the Antarctic ozone layer has now begun to occur during the month of September (Figure 2). This is not yet true for October where dynamical variability is too large relative to chemical recovery. Fingerprints of September healing since 2000 include (i) increases in ozone column amounts, (ii) changes in the vertical profile of ozone concentration, and (iii) decreases in the areal extent of the ozone hole. Along with chemistry, dynamical and temperature changes have contributed to the healing but could also represent feedbacks to chemistry. Volcanic eruptions have episodically interfered with healing, particularly during 2015, when a record October ozone hole occurred after the Calbuco eruption. Our results underscore the combined value of balloon and satellite ozone data, volcanic aerosol measurements, and a chemistry-climate model in documenting progress in the recovery of the ozone layer since the Montreal Protocol.

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Figure 2. Model trends in total ozone abundance (TOZ) by month from 2000 to 2014. Error bars denote 90% statistical confidence intervals. Contribution to the simulated monthly trends in TOZ driven by dynamics and temperature, chemistry only, and volcanoes.

PUBLICATION:

Solomon, S., D. J. Ivy, D. E. Kinnison, M. J. Mills, R. R. Neely III, and A. Schmidt, Emergence of healing in the Antarctic ozone layer, Science, 353, 269-274, 2016.